In the last few years there has been a large development in microelectronic technology, which has led to the emergence of sensor networks and various mobile applications with wireless connections. The creation of these autonomous systems has sparked a current growing interest internationally in the development of generators providing them with energy. Power to this kind of systems is normally provided from batteries, which present limitations associated with the need of recharge as well as cost. In addition, the batteries contain highly polluting elements and require recharge. Batteries also determine in many cases the final volume of the systems, constituting a limitation to the procurement of systems of increasingly reduced dimensions and which must be autonomous in operation. The need for alternative power sources other than batteries has provided for the emergence of a large number of research projects based on micro/nano systems technology for the energy generation through alternative sources of higher duration (hydrogen combustion micro batteries) or environment sources not needing recharge (light, vibration, thermal gradient). One of the most interesting options consists of taking advantage of mechanical energy present in the environment in the shape of vibrations and/or movement, due to obtaining the necessary power density to feed a micro system without depending on the level of illumination.
To make the conversion from vibration energy to useful electrical energy, different transduction principles may be used, such as: piezoelectric, electrostatic and electromagnetic. Among these different types of generators, the electromagnetic are the ones allowing for a high electromechanical connection through the use of relatively easy designs leading to high power densities.
In the case of magnetic generators with permanent magnets, the generated power density inducing voltage in a coil is fully scalable. The generators may be classified in three groups:
a) Rotational Generators
Imitate the operating method of macro-scale motors and generators. They depend on a constant source of mechanical rotational energy (e.g. fluid propelled turbine, motor . . . ). They are devices that, if provided with small dimensions, generally operate at high rotational speeds and therefore have high electrical frequencies, which enable them to match or even exceed the power density of their macroscopic counterparts.
b) Vibration Generators
They are attractive because they are able to create energy without the need of a driving force (motor, turbine). They operate at low frequencies and low power densities. Their operation is based on a magnet and a coil moving relative to the magnet due to the influence of external vibration. Maximum power generated occurs when the external vibration coincides with the device's resonance frequency.
There are different research groups on the field of inertial electromagnetic generator development using macro-components. These devices are based on the implementation of a “velocity damped resonator” and their typical volumes are of a few cm3 and may generate power ranging from hundreds of μW to mWs. In some cases the feasibility of these devices has been proved for the autonomous operation in simple systems (equipped with a sensor device and a transducer (transmitter-receptor) of data through the air).
c) Hybrid Generators
They are vibration generators that may generate energy thanks to rotational machines. Their function is to widen the range of operational frequencies in vibration generators.
The developed devices reported in literature are normally based on the implementation of designs including an eccentric magnetic rotor and a fixed coil, which enables rotation of the magnet from low frequency movements. Therefore they are adequate for taking advantage of energy associated with frequencies in the range of hertz, and a large amplitude such as those induced by waves. The designs reported in the literature enable reaching, at this time, power densities in the 10−6 W/cm3 to 10−3 W/cm3 range.